Electrophotographic image forming apparatuses known in the art include laser printers and light-emitting diode (LED) printers. An image forming apparatus includes developer cartridges and a drum unit. The drum unit includes a plurality of photosensitive drums. The developer cartridges are removable from the drum unit. When a developer cartridge is attached to the drum unit, a developing roller of the developer cartridge contacts the corresponding photosensitive drum of the drum unit.
Each developer cartridge includes a developing roller shaft rotatable together with a developing roller. When the developer cartridge is attached to the drum unit, the developer cartridge rotates, due to weight of the developer cartridge, about the developing roller shaft relative to the drum unit. The developer cartridge is then positioned relative to the drum unit.